Field of the Invention
This invention relates to video signal dropout compensation.
For many years, color television (T) signals have been transmitted using the NTSC, PAL and SECAM color systems. Due to the huge amounts of monochrome equipment in existence when these three systems were initially contemplated, economic and bandwidth considerations imposed a large restraint on their designs. Specifically, the systems had to be compatible with existing monochrome systems in that they had to transmit their signals over existing channels and in that their signals could be received in (monochrome) on existing monochrome receivers as well as being received (in color) on color receivers. As a result, the systems were so designed that the chrominance (color) information was transmitted within the same frequency band as the luminance information, the chrominance and luminance information or signals being combined to form a so-called composite signal. This is possible by virtue of the fact that, by modulating the chrominance information onto a color sub-carrier of a precisely controlled frequency, it is possible to interleave the frequency spectra of the chrominance and luminance signals so that they suffer minimal interference (cross-talk) with one another. In practice, a certain amount of cross-talk (cross-color) and (cross-luminance) does in fact occur, at least in some circumstances.
In more recent times, with the advent of direct broadcast by satellite (DBS) systems, which are not subject to the constraint of having to be sent by existing (terrestrial) channels, the MAC (multiplexed analog components) system was designed. There are several variants of the MAC system, including normal definition MAC, widescreen MAC, high definition MAC, B-MAC, C-MAC, D-MAC, D2-MAC, and so on. All of the variants are characterized by the fact that, instead of being sent in composite form, the chrominance and luminance signals are sent sequentially, that is on a time division multiplex basis, so that they cannot interfere with each other. Thus, cross-color and cross-luminance cannot occur.
A full description of the MAC system can be obtained from various published documents, including the following specification published by the European Broadcasting Union (EBU): "Specification of the systems of the MAC/packet family--Tech 3258-E", EBU Technical Centre, Brussels, October 1986. For present purposes, the relevant features of the MAC system are as follows. Each line (that is, each line scanning interval) of the transmitted signal includes time-compressed chrominance information and time-compressed luminance information, the chrominance and luminance information being sent one after the other. In similar manner to existing systems, the chrominance information comprises two color difference signals. However, the two color difference signals are not both sent during each line. Rather, to reduce transmission bandwidth, the respective two color difference signals are sent on a line sequential basis, that is during alternate lines. Thus, odd-numbered lines of a frame will contain one of the color difference signals and even-numbered lines of the frame will contain the other of the color difference signals. However, since the number of lines per frame is odd, for example 625 for normal definition MAC, to simplify the design of equipment used in the system the sequence of the color difference signals is reset between frames so that each of the color difference signals is always sent during lines of the same number of successive frames. That is, the (n)th line of each successive frame will always include one of the color difference signals and the (n+1)th line of each successive frame will always include the other of the color difference signals.
At present, with the MAC system being fairly new, most studio equipment for use in MAC studios is of a conventional type, the signal being put into MAC format immediately before transmission. However, as the use of MAC systems becomes more extensive, it is probable that studio equipment specially designed to encode or convert signals into MAC format and/or to handle signals in MAC format will become generally avaiable. In particular, to take one example, it is probable that video tape recorders (VTRs) that can store and reproduce a signal in MAC format will be widely used.
A well-known problem associated with VTRs is that of dropout, namely short intervals in which a signal reproduced from a VTR (or, more generally, a tape player) is lost, or so severely attenuated as effectively to be lost, due to tape or playback head defects. Techniques for compensating for (concealing) dropout in conventional color systems, by substituting other, similar video information for information lost due to dropout, are well established. However, due to the radically different way in which the color and luminance information is arranged in MAC signals, the known techniques are not readily adaptable for use in compensating for dropout in MAC systems or, more generally, in line sequential color systems.